This invention relates to an improvement in a vertical type pulverizing and classifying apparatus comprising a pulverizing chamber, a pluverizing rotor mounted in the pulverizing chamber to be rotatable on a vertical axis, a material feeder and a material carrying gas feed duct both connected to the pulverizing chamber, a guide ring mounted in the pulverizing chamber and directly over the rotor to be substantially coaxial with the vertical axis, the guide ring defining a gas stream ascending passage circumferentially thereof and a gas stream descending passage inwardly thereof, a classifying blade rotor rotatable substantially on the vertical axis to provide a secondary classification of fine particles of a material under treatment resulting from a primary classification effected in the gas stream descending passage, a collecting passage for removing fine particles separated out by the classifying blade rotor, and a return passage for returning coarse particles to the pulverizing chamber.
In the apparatus having such a construction, the material introduced into the pulverizing chamber by the material feeder is pulverized by the pulverizing rotor therein and the pulverized material is carried by gas streams entering from the gas feed duct, through the gas stream ascending passage defined outwardly of the guide ring and through the gas stream descending passage inwardly thereof, undergoing the primary classification effected by the gas streams in the gas stream descending passage. Fine particles of the material resulting from the primary classification are subjected to the secondary classification effected by the rotation of the classifying blade rotor, and resulting fine particles are removed by way of the collecting passage whereas coarse particles are returned to the pulverizing chamber by way of the return passage.
An example of known classifying apparatus of this type is disclosed in U.S. Pat. No. 3,285,523 (or Japanese Patent Publication No. 50-21695). This known apparatus is described first with reference to FIG. 2 of the accompanying drawings. As seen, the apparatus has a classifying blade rotor 13 disposed inwardly of a guide ring 5. Material introduced by a feeder 4 is pulverized by a rotor 1, and resulting particles of the material are carried through a gas stream ascending passage 8 and then through a gas stream descending passage 9 by gas introduced from a gas feed duct 10. Fine particles resulting from a primary classification effected in the gas stream descending passage 8 are subjected immediately to a secondary classification effected by the action of the classifying blade rotor 13. Fine particles that pass through the classifying blade rotor 13 are removed by way of a collecting passage 14 whereas coarse particles emerging from the primary and secondary steps of classification are immediately returned to a pulverizing chamber 3.
According to this prior art arrangement, the primary classification and secondary classification are carried out in the same space, producing an unfavorable effect on each other to the detriment of classifying precision and efficiency. A further disadvantage of the known apparatus is that, since all of the coarse particles emerging from the classification are immediately returned to the pulverizing chamber, the pulverizing rotor 1 must operate under great load variations and is thus prone to bad pulverizing efficiency.